1. Field of the Invention
This invention relates to an EL (electro-lumninescence) display device formed by constructing a semiconductor device (i.e., a device made of a semiconductor thin film) on a substrate and relates to electronic equipment (electronic device) in which the EL display device is used as a display panel (display portion).
2. Description of Related Art
In recent years, great advances have been made in a technique for forming TFTs on a substrate, and development has proceeded in the application thereof to an active matrix type display. Especially, a TFT using a polysilicon film is higher in electron field-effect mobility than a conventional TFT using an amorphous silicon film, and can operate at a high speed. Therefore, it has been made possible to control a pixel by a driving circuit formed on the same substrate on which the pixel is also formed, although the pixel had been conventionally controlled by the driving circuit disposed outside the substrate.
The active matrix type display is attracting public attention because it can obtain various advantages, such as reduced manufacturing costs, reduced size of the display device, increased yields, and reduced throughput, by constructing various circuits or elements on the same substrate.
Conventionally, the pixel of the active matrix type EL display has been generally constructed as shown in FIG. 3. In FIG. 3, reference character 301 designates a TFT that functions as a switching element (hereinafter, referred to as switching TFT), 302 designates a TFT that functions as an element (current controlling element) to control a current supplied to an EL element 303 (hereinafter, referred to as current controlling TFT), and 304 designates a capacitor (capacitance storage). The switching TFT 301 is connected to a gate wiring line 305 and a source wiring line 306 (data wiring line). The drain of the current controlling TFT 302 is connected to the EL element 303, and the source thereof is connected to a current-feed line 307.
When the gate wiring line 305 is selected, the gate of the switching TFT 301 is opened, the data signal of the source wiring line 306 is then stored in the capacitor 304, and the gate of the current controlling TFT 302 is opened. After the gate of the switching TFT 301 is closed, the gate of the current controlling TFT 302 is kept opening by the charge stored in the capacitor 304. During that interval, the EL element 303 emits light. The amount of luminescence of the EL element 303 changes according to the amount of a flowing current.
At this time, the amount of current supplied to the EL element 303 is controlled by the gate voltage of the current controlling TFT 302. This is shown in FIG. 4.
FIG. 4(A) is a graph showing transistor characteristics of the current controlling TFT. Reference character 401 is called Id-Vg characteristic (or Id-Vg curve). Herein, Id is a drain current, and Vg is a gate voltage. The amount of a flowing current corresponding to an arbitrary gate voltage can be known from this graph.
Normally, the region shown by the dotted line 402 of the Id-Vg characteristic is used when the EL element is driven. An enlarged view of the enclosed region of the dotted line 402 is shown in FIG. 4(B).
In FIG. 4(B), the region shown by the oblique lines is called a sub-threshold region. In practice, it is indicated as a region in which a gate voltage is near or less than a threshold voltage (Vth). The drain current exponentially changes according to the change of the gate voltage in this region. Using this region, the current is controlled by the gate voltage.
The data signal input into a pixel by opening the switching TFT 301 is first stored in the capacitor 304, and the data signal directly acts as the gate voltage of the current controlling TFT 302. At this time, the drain current with respect to the gate voltage is determined by one-to-one according to the Id-Vg characteristic shown in FIG. 4(A). That is, a given current flows through the EL element 303 corresponding to the data signal, and the EL element 303 emits light by the amount of luminescence corresponding to the amount of the current.
The amount of luminescence of the EL element is controlled by the data signal, as mentioned above, and thereby gradation display is performed. This is a so-called analog gradation method, in which the gradation display is performed by a change in the amplitude of the signal.
However, there is a defect in that the analog gradation method is very weak in the characteristic variability of TFTs. For example, let it be assumed that the Id-Vg characteristic of a switching TFT differs from that of a switching TFT of an adjacent pixel that displays the same gradation level (i.e., a shift is performed toward a plus or a minus side overall).
In this situation, drain currents of the switching TFTs differ from each other, though depending on the level of the variability, and thus a different gate voltage will be applied to the current controlling TFT of each pixel. In other words, a different current flows through each EL element, and, as a result, a different amount of luminescence is emitted, and the display of the same gradation level cannot be achieved.
Additionally, even if an equal gate voltage is applied to the current controlling TFT of each pixel, the same drain current cannot be output if the Id-Vg characteristic of the current controlling TFTs has variability. Additionally, as is clear from FIG. 4(A), a region is used in which the drain current exponentially changes according to a change in the gate voltage, and, therefore, a situation will occur in which, if the Id-Vg characteristic shifts most slightly, the amount of current to be output becomes greatly different even if an equal gate voltage is applied thereto. If so, adjacent pixels will have a great difference in the amount of luminescence of the EL element.
In practice, each individual variability of the switching TFT and the current controlling TFT acts synergistically, and a stricter condition will be imposed. The analog gradation method is extremely sensitive to the characteristic variability of the TFTs, as mentioned above, and this has caused an obstruction to realizing the multicolor of the conventional active matrix type EL display device.